The present invention relates generally to the field of methods of and system for capturing fingerprint images, and more particularly to a narrow array capacitive semiconductor fingerprint imaging system.
Fingerprint recognition has been suggested for use in many security applications, such as controlling access to buildings, computers, or the like. Fingerprint recognition systems enable a user to access the controlled facility without having to provide a device such as a keypad or card reader, and without having the user memorize a password, or other personal identification number, or carry a card key.
The sensing device is an important part of a fingerprint recognition system. The quality of the fingerprint image that the sensing device produces will affect recognition capability and the amount of processing required for verification of the fingerprint.
Various technologies have been proposed for use in fingerprint sensing devices. One commonly proposed technology involves optical image detection. Examples of optical fingerprint detection devices are disclosed in Jensen, U.S. Pat. No. 4,784,484; Fishbine, et al., U.S. Pat. No. 5,467,403; and Giles, et al., U.S. Pat. No. 5,548,394.
Optical detectors include a glass surface upon which a subject places his finger to be recognized. Optical detectors may present recognition problems when the glass surface or the subject""s finger is wet. The optics of the detectors are constructed based upon the indices of refraction of air and glass. When water or perspiration is between the glass and surface of the finger, the operation of the detector is affected. Additionally, optical detectors may be xe2x80x9cspoofedxe2x80x9d by placing an image of a valid fingerprint on the glass surface.
In addition to optical sensors, various electrical sensor systems have been proposed, as for example in Knapp, U.S. Pat. No. 5,325,442; Tamori, U.S. Pat. No. 5,400,662; and Tamori, U.S. Pat. No. 5,429,006. The electrical detection devices typically comprise an array of sense elements. The individual sense elements respond with an output that depends on whether a fingerprint valley or ridge is located over the sense element.
The electrical detection devices offer some advantages over the optical detection devices. The optics of the optical devices can be larger and expensive. The electrical detection devices are less subject to the moisture problems discussed with respect to the optical devices. Also, electrical detectors sense based upon a three-dimensional fingerprint model, making them less subject to spoofing with a two-dimensional fingerprint image. However, electrical detectors are subject to scratching and electrostatic discharge, thereby having some disadvantages in terms of robustness. Also, most electrical fingerprint detectors are based upon large arrays of sensing elements and therefore can be expensive.
An example of a larger array electrical sensing device is the TouchChip (TM) Silicon Fingerprint Sensor, which is available from STMicroelectronics, Inc. The TouchChip sensor uses an active pixel array based upon a capacitive feedback sensing circuit of the type disclosed in Tartagni, U.S. patent application Ser. No. 08/799,548, filed Feb. 13, 1997, titled Capacitive Distance Sensor. The array of the TouchChip sensor comprises 364 rows and 256 columns of cells, representing 93,184 pixels, and having dimensions of about 18.2 mm by about 12.8 mm. Each pixel cell contains a high-gain inverter connected to two adjacent top metal plates separated from the skin surface by an ultra-hard protective coating. The inverter input is connected to one of the top metal plates and the inverter output is connected to the other top metal plate. The cell provides a charge integrator whose feedback capacitance is the effective capacitance between the two top metal plates.
When a finger is placed on the sensor, the surface of the skin over a pixel cell acts as a third plate separated from the two adjacent plates by a dielectric layer composed of air. Because fingerprint valleys will be farther from the sensor surface than fingerprint ridges, pixel cells beneath valleys will have more distance between their top metal plates and the skin surface than pixel cells under ridges. The thickness of the dielectric layer modulates the capacitive coupling between the top metal plates of the pixel cell so that top metal plates under valleys will exhibit different effective capacitance than top plates under ridges.
In Kramer, et al., U.S. Pat. No. 6,317,508 titled Scanning Capacitive Semiconductor Fingerprint Detector, there is disclosed a narrow array capacitive fingerprint detector. The Kramer, et al. detector comprises an array of capacitive pixel cells of the type disclosed in Tartagni. The array of Kramer, et al. is about 256 by about 20 to 50. Accordingly, the array has a first dimension that is about the width of a fingerprint and a length that is substantially less than the length of a fingerprint. The Kramer et al. device captures a fingerprint image as the subject sweeps the fingertip over the narrow array as the array is scanned on each scan, the narrow array captures a partial image or slice of the fingerprint. A regeneration algorithm assembles the slices into the complete fingerprint image.
One of the problems with a narrow array device is that the speed at which the finger is swept over the array is unknown. In order to reconstruct the fingerprint image, a pair of consecutive slices must have enough rows in common for them to be aligned by the regeneration algorithm. The more rows in common from one slice to the next, the more exactly the regeneration algorithm can combine two slices into a single larger slice. Thus, the fingerprint image must be over-sampled. Since different people sweep their fingers at different speeds and the speed at which a person moves the finger during any particular sweep is not generally not uniform, the fingerprint image must be substantially over-sampled.
The narrow array must be scanned at a relatively high clock rate to ensure that the fingerprint image is sufficiently over-sampled. Additionally, the number of rows in the narrow array must be sufficient, for a given clock rate, to ensure that the fingerprint image is sufficiently over-sampled. The over-sampling required for accurate image reconstruction requires substantial memory. Finally, the regeneration algorithm required for reconstructing the image requires processor resources.
Thus, although a narrow array device is less expensive to build than a large array device, narrow array devices may be more expensive in terms of memory and processing resources.
The present invention provides a narrow array capacitive semiconductor fingerprint detection system that captures a plurality of partial fingerprint images and assembles the partial images into a complete fingerprint image substantially without over-sampling. The system of the present invention includes an array of capacitive sensing elements. The array has a first dimension about the width of a fingerprint and second dimension less than the length of a fingerprint. A scan control unit is coupled to scan the array at a scan rate determined by the speed of finger movement over the array. The scan control unit scans the array capture, partial fingerprint images. Output logic is coupled to the array to assemble the captured fingerprint images into a complete image based upon the direction of finger movement over the array. A mouse device is positioned adjacent the array in the path of finger movement over the array. The mouse device is coupled to provide finger movement speed information to the scan control unit. The mouse device is also coupled to provide finger movement direction information to the output logic.